It should be well known amongst the few of you that read this that I am a very light reader.  I enjoy it when I can find the kind of solitude that the main character from I AM LEGEND is privy to (one of the very few benefits of being the last man on earth), but I lack the patience to space out a 400 page novel to be completed within a week's time.  A week is what I'd give myself to complete a book, otherwise I'd feel ashamed that I am an inferior reader, which I am because I don't read.  So, either I read something and take my time, but feel inadequate, or I read something rapidly and not completely recall the words on the page.  It's a vicious decision, so I go about the lazier option and just not do it at all, much to my own dismay.
This has no bearing whatsoever on my thoughts of the films, other than to inform you that I haven't read the novel.  So, what I think of each of the different screen adaptations (THE LAST MAN ON EARTH, THE OMEGA MAN, and I AM LEGEND) is from the viewpoint of someone who knows the story as it's told from the different films and not from the original source.  There are certainly similarities between each of the films, but it's always the little differences that can turn the tide in favor of one over another.


THE APOCALYPSE

As you may have noticed from the trailers to the most recent adaptation, I AM LEGEND starring one of the more entertaining action movie actors of his generation, Will Smith, something cataclysmic on a worldwide scale occurred leaving just one man alive.  But, as you may also have noticed from the trailer, and I quote, "The last man on earth, is not alone..."
So, something monumentally devastating happened leaving just one human being, but in turn also created something completely new.  It is now up to this one man to not only survive as the last remaining human, but also to find a solution to the catastrophic problem that brought upon the apparent end of the world.  Luckily, the last remaining human is a scientist.

THE INFECTION

The cataclysmic event that I spoke of was a deadly viral outbreak.  It's essentially the same in all three films, only the cause of the disease and the symptoms differed.  It's a virus that can infect others by airborn attack, or by physical contact from someone already infected.  The main character in each of the films found himself immune to the virus, for different reasons but none so preposterous as to exclude one from the group.
The cause of the virus in THE LAST MAN ON EARTH, the first screen adaptation of the novel starring horror movie icon Vincent Price, was that it was a strain that originated in Europe and was carried with the wind to our homeland.
In the 1970s adaptation THE OMEGA MAN, starring action film legend Charlton Heston, the cause of the outbreak went unknown to the viewers.  All we knew was that Heston was somehow immune, but we find out later on why.
In the 2007 version of the story, things get more in-depth and scientific.  The virus is the aftermath of an initially successful treatment for cancer.  Cancer patients were given a proven syrum that was created by using another disease, the measles, to fight off the cancer cells.  The cure was successful, until it spawned a supposed incurable strain that went airborn and wiped out the earth's population.

THE INFECTED

Here's where things start to get interesting between the different screen versions of the novel.  With each new adaptation of the novel the infected people grew increasingly powerful in physical strength, more prominent in the latest adaptation which I think is the major problem with the film.  More on that later.
In THE LAST MAN ON EARTH the infected people represented a combination of zombie-like and vampire-like beings.  They're probably the lowest creation on the horror film food chain in terms of the threat.  They move at the pace of a classic Romero zombie, and inherited the same level (or lack) of intelligence, while also being sensitive to sunlight.  They essentially get the worst of both worlds without getting the perks of being a vampire.  It's sort of a more "realistic" representation, I would think, of people being infected by what should be a sickness.  When you're sick you typically feel worse, and I can't imagine feeling any worse than the people in THE LAST MAN ON EARTH do.
In THE OMEGA MAN they've done away with the zombie symptoms, and don't seem to be cannibalistic.  They're really just humans that steer clear of UV rays, and are only symptomatic as far as becoming a skin-cracked albino with an increased gullibility for false prophets.  The zombie aspect of the infected people is actually a bit more metaphorically represented than literally, as the infected people keep their human traits but act as a race of people incapable of questioning authority, with an exception applied to the leader.  It's similar to that of hardcore evangelists (no insult intended).
Finally, in I AM LEGEND the idea of this working as a sickness has been almost completely neglected.  Apparently, when you try and cure cancer with another disease you're able to break the barriers of reality, and you become a compter generated Taz the Tazmanian Devil.  Yes, I'm slightly exaggerating, but only slightly.  By combining cancer and the measles into one human body, you can create a human with ape-like strength, cat-like reflexes and agility, but you lose your thinking capacity and become something just more animalistic without the ability to walk in sunlight.  In fact, you can make a case that the main night walker in the film may have been the next evolutionary being as he showed signs of intelligence with an almost endless pain threshold.  
In all honesty I don't know if I prefer any of these representations over another.  I think I prefer the approach of THE LAST MAN ON EARTH, but I'd wished they were as human as the infected from THE OMEGA MAN, but I can't blame a 50s drive-in movie for being a 50s drive-in movie.  The super-human sickly's from I AM LEGEND certainly make for a much higher degree of pulse-pounding moments, but they are counter-productive to the more superior dramatic and solitary elements of I AM LEGEND over the other two movies.  
Insert the "zombies" from Danny Boyle's 28 DAYS LATER into any of these movies and it raises the quality of each of them drastically.

THE INTERACTION

Where I really prefer I AM LEGEND to the other two is in its choice to exclude unnecessary dialogue.  One of the major problems I have with THE LAST MAN ON EARTH and THE OMEGA MAN (to a lesser degree) is that they both feel the audience is incapable of sitting through long periods of silence, and that we're incapable of figuring things out for ourselves.  It's not insulting, but I think it significantly detracts from my willingness to connect with the main character if he just talks to himself about things that he has no real business talking to himself about.  Watching THE LAST MAN ON EARTH on mute would probably solve the problem as Vincent Price does only speak through internal monologues, which I like, but the things he says to himself are the types of things that are only there for him to say in order to blatantly tell the audience why he's doing what he's doing.  You don't have to tell me that the infected people hate mirrors and garlic, I can deduce that on my own when I see there's a broken mirror and a necklace of garlic on the front door.  I may not necessarily know exactly why they're there, but I'd assume they're there to work in the favor of our hero.  After all, he is a scientist.  Will Smith does something similar in I AM LEGEND as he tosses a gallon of liquid on his front doorsteps.  He doesn't say something like, "I'm glad I found out about this liquid covering my scent, otherwise we'd be screwed..." because I can figure out for myself that he's not doing it for the hell of it.  I still don't know what liquid he used, and what the reason for it was, but I can pretty much conclude that it's probably something to cover his tracks and stay hidden.  That's all that I need to know, and I don't need him to give me the A to Z explanation by speaking unnatural dialogue, whether it be internal or external.  Any person in that situation would not say something like that to themself.
I also love the inclusion of the dog to act as a sidekick, because the audience as a whole may in fact be strained to sit through as much as silence as this type of movie would have.  It's a much better solution than unnecessary speech, and it also relieves a lot of pressure off of Will Smith to find something within himself to be a damaged individual.  This is probably one of Will Smith's better performances, but I do think that had he not had the dog to interact with that he could have buckled under the pressure of carrying an entire film without being able to use his natural gifts of charm.  There are a lot less actors out there that I think can remain interesting by representing their emotional state physically and not verbally.  It also didn't hurt that he turned a set of mannequins into his own group of CAST AWAY Wilsons to converse with.


THE LEGEND

Honestly, I think Will Smith had the best material to work from, and therefore he, and his film get the bid.  The two prior films seemed more products of their time, and I think the possibility of any of them surviving the longest heavily favors I AM LEGEND.  THE LAST MAN ON EARTH is primarily 50s B-movie drive-in fare, and falls prey to having a less than threatening threat.  It's more a zombie film than an apocalyptic film.
THE OMEGA MAN, also a product of its time, is not a horror film nor does it portray the sense of global destruction.  It's an action film that won't feel like anything else thanks to a very sour choice in music.  It utilizes a prodominently typical 70s action-film score instead of something a little more brooding to give a sense that this is basically the end of the world.  A John Carpenter score could have done wonders for this film as he seems to know the right keys to express grave danger.
I AM LEGEND is as much a product of today as the other films are a product of their day.  I don't like the choice to use computer generated monsters, but I can say that it was certainly more terrifying than the other depictions, just not the depiction of preferance for my tastes.  If it's computer generated then it isn't anything of real substance, therefore is near impossible to feel real, and therefore I'm not scared.  However, overall I think the approach to I AM LEGEND was a bit more mature than its predecessors, both with the storyline and with the material given to the main actor.  Vincent Price suffered from having constant unimportant self speeches, and Charlton Heston suffered from having sub-standard things to say.  Will Smith was given a more emotional part, probably his most demanding role, and succeeded with his performance.


THE CONSENSUS

I guess only time will tell if I AM LEGEND holds up.  It's arguable whether or not  the others do.  I still enjoy them because I enjoy the types of films they are and recognize the period they were released in, but also realize they have a slim chance of finding an audience that won't laugh at a lot of the things that are significantly dated.  Even though they're all essentially the same story, I think there are more timeless elements found in I AM LEGEND, both in the filmmaking and in the adaptation.

Even though I haven't read the novel I can tell that there's a much better version of this story waiting to be told.  The different parts of the story have been filmed much better, and recently, just not all in the same picture.  I'm assuming that there's supposed to be something akin to a combination of 28 DAYS LATER and CHILDREN OF MEN that we should be seeing, but haven't had the right person behind the camera to make it.   
Even if that's not what the story is supposed to be like, I can't imagine the source material of I AM LEGEND being able to produce anything superior to that.  Maybe 28 MONTHS LATER will be the best I AM LEGEND ever made.